A valve mechanism as set out above is known from JP-2004360485. In this prior art reference, a post with an oil passageway projects from the cylinder head. A bridge assembly straddling the stems of the two poppet valves includes a cylinder housing. The post which projects from the cylinder head passes through a hole in the lower end of the cylinder and guides the bridge assembly to slide parallel to the post and to the valve stems. An annular piston, which serves the purpose of the slider mentioned above, is disposed within the cylinder and is moved by the cam that operates the two valves. The outer wall of the piston seals against and slides relative to the inner wall of the cylinder and its inner wall seals against and slides relative to the outer wall of the post. A working chamber is defined within the piston that receives hydraulic fluid (engine oil) through the passageway in the post. The pressure in the working chamber operates a locking system which selectively locks the piston to the bridge.
In this way, when the piston is locked to the bridge the valves are opened and closed by the cam whereas when the locking system is released, the annular piston moves with the cam but is uncoupled from the bridge so that the valves remain stationary. This allows the valves to be deactivated hydraulically.
To maintain the annular piston in contact with the cam and the bridge in contact with the valve stems, a spring is housed within the cylinder, surrounding the post, to urge the piston and the cylinder apart.
The present invention recognises certain problems with such a construction. In order to be able to push the piston against the cam and prevent the bridge from lifting off the poppet valves the spring needs to be fairly substantial. A first problem in meeting this requirement is that of space. The maximum dimensions of the cylinder in the bridge assembly are determined by the limited space between the poppet valves and it is difficult to provide enough space in the cylinder to accommodate a spring of the size required. Furthermore, if a large spring is used and the bridge assembly is somehow enlarged to accommodate it, then this results in a significant increase in the inertial mass of the bridge assembly and larger valve springs are then required to cope with this increased mass that the valves have to lift as they close.